The reality is different. It is more common for skins to drift out of level and the problem is known by many names in addition to the above!
The use of conventional DPC within a cavity wall at changes of level and abutments can be problematic at the best of times, but when the masonry skins are out of alignment the position is even worse.
Such problems can be banished by using warranted preformed Cavitrays. Approved preformed Cavitrays are self-supporting so require no support from the inside skin. At what height the inside skin coursing occurs makes no difference and if skins are out of alignment, preformed Cavitrays still function. The same applies if the cavity width varies. Cavitray upstands are hinge-biased to take up the as-found cavity dimension, promoting compatibility and conformity. Importantly, as every tray is moulded and thus correctly profiled, there need be no arisings of distortion, sagging, mis-shaping, damage or puncturing.
When Cavitrays are used with metal, stone or concrete lintels, one eliminates the variances that occur with roll DPC cut and shaped on site. Preformed Type C Cavitrays always harmonize with the window head construction / lintel choice, so the build detail is always consistent, always correct. Both skins remain independent of each other.
Importantly, Type C Cavitrays for use with all styles of lintel have extended ends that project beyond the wall opening by 300mm each side. These extended ends serve two purposes: Firstly, they provide adequate shelter of whatever reveal closing method is deployed. Secondly, the 300mm projection ensures a naturally-occurring brick perp joint beyond the opening width is always located to accommodate the stopend in the optimum position. Accordingly, if lintels with only 150mm bearings (such as those within the 900 to 1600mm length range) are used the 300mm tray continuance always secures a perp in the right place. (Acknowledging with standard bricks the first perp can beyond the lintel end.
Finally, preformed Type C Cavitrays can be supplied with a drip lip moulded to the tray front in a position to coincide with the top of the window or door frame. Thus, the recommendation within NHBC 6.1D6b to provide a lip at this point is fulfilled automatically.
Cavity Trays of Yeovil offers Cavitray scheduling and delivers trays packaged per plot, so there is no waste and stocks and costs are controlled. Importantly, the builder benefits a consistent construction detail – every time.
Preformed Cavitrays are accompanied with a performance warranty and can design out risk by banishing pigs and gorrets. Cavity Trays is based in Yeovil, Somerset and is the only UK tray manufacturer awarded European Technical Approval.
There’s an enlightening ‘CPD silent study’ that looks at horizontal and curved head openings. It can currently be accessed free on the Cavity Trays website.